TST: adds pandas.util.testing.patch

This function temporarily replaces a an attribute on an object for the
duration of some context manager. This is useful for patching methods to
inject assertions or mock out expensive operations.

Author: Joe Jevnik

pandas/io/tests/test_packers.py

@@ -1,6 +1,5 @@
 import nose
 
-from contextlib import contextmanager
 import os
 import datetime
 import numpy as np
@@ -16,7 +15,8 @@
 import pandas.util.testing as tm
 from pandas.util.testing import (ensure_clean, assert_index_equal,
                                  assert_series_equal,
-                                 assert_frame_equal)
+                                 assert_frame_equal,
+                                 patch)
 from pandas.tests.test_panel import assert_panel_equal
 
 import pandas
@@ -59,20 +59,6 @@ def check_arbitrary(a, b):
         assert(a == b)
 
 
-@contextmanager
-def patch(ob, attr, value):
-    noattr = object()  # mark that the attribute never existed
-    old = getattr(ob, attr, noattr)
-    setattr(ob, attr, value)
-    try:
-        yield
-    finally:
-        if old is noattr:
-            delattr(ob, attr)
-        else:
-            setattr(ob, attr, old)
-
-
 class TestPackers(tm.TestCase):
 
     def setUp(self):

pandas/util/testing.py

@@ -2243,3 +2243,55 @@ class SubclassedDataFrame(DataFrame):
     @property
     def _constructor(self):
         return SubclassedDataFrame
+
+
+@contextmanager
+def patch(ob, attr, value):
+    """Temporarily patch an attribute of an object.
+
+    Parameters
+    ----------
+    ob : any
+        The object to patch. This must support attribute assignment for `attr`.
+    attr : str
+        The name of the attribute to patch.
+    value : any
+        The temporary attribute to assign.
+
+    Examples
+    --------
+    >>> class C(object):
+    ...     attribute = 'original'
+    ...
+    >>> C.attribute
+    'original'
+    >>> with patch(C, 'attribute', 'patched'):
+    ...     in_context = C.attribute
+    ...
+    >>> in_context
+    'patched'
+    >>> C.attribute  # the value is reset when the context manager exists
+    'original'
+
+    Correctly replaces attribute when the manager exits with an exception.
+    >>> with patch(C, 'attribute', 'patched'):
+    ...     in_context = C.attribute
+    ...     raise ValueError()
+    Traceback (most recent call last):
+       ...
+    ValueError
+    >>> in_context
+    'patched'
+    >>> C.attribute
+    'original'
+    """
+    noattr = object()  # mark that the attribute never existed
+    old = getattr(ob, attr, noattr)
+    setattr(ob, attr, value)
+    try:
+        yield
+    finally:
+        if old is noattr:
+            delattr(ob, attr)
+        else:
+            setattr(ob, attr, old)